1. Field of the Invention
The invention concerns a device for cooling the power electronics integrated at the rear of a reversible electric machine such as an alternator or an alterno-starter of an automobile. The invention has applications in the automobile industry and, in particular, in the area of alternators and alterno-starters for automobiles.
2. State of the Technology
In an automobile, the alternator can transform the rotating movement of a winding rotor, driven by the thermal motor of the vehicle, into an induced electric current in the field coil of the polyphased stator. In general, this stator is comprised of three phase windings, so that the alternator is the triphased type. The three phases of the armature are connected to a bridge rectifier. This bridge rectifier has three branches, each comprised of at least two diodes connected to each phase. Now, these diodes generate heat. Traditionally, the bridge rectifier can dissipate energy of about 150 watts. Therefore, it must be cooled to avoid any overheating of the diodes.
FIG. 1 shows an example of the rear portion of a conventional alternator. This alternator has a rotor 1 mounted on a rotating shaft 2, and the axis of rotation is referenced A and forms the axis of the machine as visible also in document DE A 197 05 228. This rotor 1 is surrounded by a stator 3 comprised of a magnetic circuit 8 and an field coil 7. The stator 3, via its circuit 8 and its winding 7 generate an alternating current. The field coil has phase windings connected in a star and/or triangle. Each of these windings has an output connected to the bridge rectifier. The current generated in stator 3 is rectified using the bridge rectifier containing the diodes 9. This stator 3 is supported by a rear bearing 4 and a front bearing (not shown). The rotating shaft 2 is held by the two bearings with rolling bearings 6. As described in document DE A 0 197 05 228, the magnetic circuit 8 comprises a stator body in the form of a packet of sheets which, as is known, contains notches, advantageously the semi-closed type, for mounting the windings of the phases crossing the body of the stator and extending on either side of the body to form chignons. In this document, one sees the front bearing of the alternator, as well as the alternator pulley, which is designed to be driven in rotation by the thermal motor of the vehicle via a transmission that has at least one belt, and the internal fans carried by the rotor and placed radially beneath the chignons, for internal ventilation of the alternator. To do this, the bearings, which are hollow, have intake and outtake outlets as described below. The rear fan, referenced as 5 on FIG. 1, is advantageously more powerful than the front fan.
In this alternator, one part of the dissipating elements, i.e. the positive diodes 9 of the bridge rectifier, are mounted in a heat energy sink bridge 10. This heat dissipating bridge has openings 10a–10d, also called outlets, in which the cooling air circulates.
The diodes 9 are connected electrically to a connector 14 which also has air passageways 14a–14f. 
In addition, the heat dissipating bridge 10 has fins 13 on the upper surface, which encourage cooling of the heat dissipating bridge 10.
More specifically, the alternator in FIG. 1 has, in the rear part, a cover 11 that surrounds and protects the power electronics of the alternator corresponding notably to the bridge rectifier. To allow the passage of air inside the cover 11, the cover is equipped with openings 12a–12d, also called outlets. These openings are placed primarily in the upper part of the cover 11. In addition, the rear fan 5 is mounted on the rotating shaft 2 or on the rotor 1 suction air inside the alternator. This fan can be the centrifuge or helico-centrifuge type, for example. In this way, the air, suctioned by the fan 5, enters the rear part of the alternator through the outlets 12a–12d and, channeled by the fins 13, hits the heat dissipating bridge 10 and the diodes 9 and, as a result, cools them. The air then leaves radially through the outlets 4a–4d installed in the rear bearing 4 of the stator 3.
Thus, the air is suctioned primarily into the axis of the alternator at the level of the protective cover 11, then it is laterally discharged through the outlets of the rear bearing 4, cooling the bridge rectifier, but also the other hot parts of the alternator, such as the chignons of the field coil 7.
For more details, the path taken by the cooling air flow is shown, on FIG. 1, by dotted lines and arrows. Refer also to document DE A 197 05 228, which describes an example of a bridge rectifier fabrication and an example of a rotor in the form of a rotor with prongs. Document DE A 101 11 295 describes another type of bridge rectifier. In these two documents, the negative diodes are supported by the rear bearing and are mounted on a plate attached to the rear bearing or fitted into the rear bearing, while the positive diodes are mounted on a plate away from the negative diodes. In document DE A 100 11 295, this plate has an opening. This plate corresponds to the heat dissipating bridge 10 in FIG. 1; the diodes 9 are the positive diodes, as described above.
Currently, there are also reversible alternators, which can form an electric motor that drives in rotation the thermal motor of the vehicle via the rotor shaft integrated with the pulley of the alternator. Such a reversible alternator is called an alterno-starter, or an alternator-starter, and transforms mechanical energy into electrical energy and vice versa. Thus, an alterno-starter can start the engine of the automobile, or act as an auxiliary motor to assist the thermal motor of the vehicle to drive this automobile vehicle.
In this case, the bridge rectifier located at the outlet of the armature of the alterno-starter, i.e. connected to each phase of the armature, also acts as a bridge to control the phases of the alterno-starter. This bridge rectifier has three branches, which each have at least two MOS-type power transistors. The transistors of this bridge rectifier are each controlled by a control unit. This control unit may be made in various ways. Most often, this control unit has a driver associated with a comparator and other electronic components. A bridge rectifier made in this way from power transistors and control units dissipates less energy than the energy dissipated by a bridge of diodes. In effect, when the bridge rectifier operates in rectifying mode, and not in control mode, then the power transistors are controlled in a synchronous way. For more information, refer for example to document EP A 1, 134,886. However, the energy dissipated is about 50 watts and the bridge rectifier must, therefore, be cooled.
Now, the control units as they have just been described are relatively large, so that the mounting of these control units and power transistors on a heat dissipating bridge leaves no room on the heat dissipating bridge for outlets. Therefore, it is not possible to cool the bridge with power transistors by air circulation as shown on FIG. 1.
In other words, the arrangement explained above requires making axial air passageways through the heat dissipating bridge and the connector, which reduces the space available to place electronic components. In fact, this available space is sufficient to place a bridge rectifier with diodes, but is not sufficient for a larger power electronics unit. In particular, in the case of an alterno-starter, the power electronics are such that each diode of the bridge rectifier is replaced by at least one transistor and one control unit.
To solve this space problem, patent application EP-A-1 032 114 proposes a cooling device for the power electronics of an alterno-starter in which the dissipating elements are comprised of a plated base on the rear bearing of the alterno-starter, and this rear bearing is comprised of channels for the passage of the cooling air. In other words, in this device, the heat dissipating bridge is placed against the rear bearing which has, on its rear outside surface, cooling fins. Thus, the air arrives laterally or radially and cools through convention first, the rear bearing with the fins and, second, the heat dissipating bridge on which the power electronics are mounted. In addition, the heat dissipating bridge is also cooled by conduction through the fins of the rear bearing with which it is in mechanical contact.
However, in such a device, it is necessary for the heat dissipating bridge, or base, to be placed securely against the bearing so that the power electronics can be cooled. In effect, if any space exists between the base and the surface of the bearing, then thermal conduction will not occur or will be poor between the base and the bearing and, as a result, there is only partial cooling of the power electronics.
In addition, if the rear bearing is very hot, it will also be difficult to cool the heat dissipating bridge by convection.